realtek: eth: cleanup packet length calculation

realtek: eth: cleanup packet length calculation

During transmit the driver must adapt the packet length. The
hardware requires at least a memory space of ETH_ZLEN bytes
data plus four bytes for layer 2 FCS. This was calculated
(somehow) but skb->len never got updated and for the minimum
length a RTL838x specific workaround was in place. Clean up
the code and use skb_put_padto() so the length change gets
reflected in skb->len.

While we are here drop zeroing DSA tag because it will be
overwritten by hardware for FCS.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://github.com/openwrt/openwrt/pull/21999
Signed-off-by: Robert Marko <robimarko@gmail.com>